Uninterruptible power supplies (UPS) designs typically work by inputting AC, rectifying the AC to DC that is coupled to a battery, and then inverting the battery output back to AC power. In the event the input AC goes away, the battery continues supplying the inverter with power until AC power returns or the battery is drained.
However, even high efficiency uninterruptible power supplies lose a lot of energy, primarily to heat, and at best may be on the order of ninety-three percent efficient in contemporary designs. This loss is highly undesirable in large data center scenarios where megawatts of power are typically needed. One solution provides a complex, customized design that bypasses the battery when AC is available. However, this is also expensive.
AC/DC rack level or in server battery backup/UPS designs create problems for high impedance power sources such as diesel generators (which are run when the regular source of AC power is lost, before the batteries are drained). One problem is that when the input voltage (VAC) drops below a specified (Under Voltage Protection, or UVP) level, the server powers supplies shut off and transfer load the local energy storage. When the generator is ready VAC input voltage increases to the point of operation, the power supply automatically turns on, the server load is removed from the local energy storage, and picked up by the power supply and then the generator. At this time, generator experiences an abrupt load increase from zero to one-hundred percent, e.g., a 2.5 MW generator needs to transition from OW to 2.5 MW with 10 msec from 10,000 250 W servers. Due the high output impedance characteristic of the generator, a high rate transient load increase causes an output voltage droop. The voltage droop is substantial enough to trip the UVP in the power supplies which summarily shut off and remove the load from the generator (one-hundred percent to zero percent transition). A continuous “on/off/on/off” or “motor boating” of energy dump is created. This behavior continues until local energy storage is depleted and cannot sustain the servers during the loss of VAC, or a component failure in the generator, distribution, or server power supply occurs.
Methods to correct this problem at the server include varying the UVP threshold, VAC good threshold, and soft start circuit delay circuits. For a server application on the order of ten thousand of servers this introduces several thousand power supply designs, each with its own turn off-and-on signature. Even then, this method is not fail proof. Further, the addition of power supply designs/part numbers increases the cost of unit production as well as warranty repair management.
A second tier control of reloading the generator is to delay the static transfer switchover. This still creates a block loading effect. Combined with variable turn off/turn on signatures, this is solution is complicated to deploy, expense to manufacture, and support.